The invention concerns a nuclear magnetic resonance (=NMR) magic-angle spinning (=MAS) probe head with an MAS stator disposed in a tube, wherein the MAS stator comprises a base bearing and a front bearing for receiving a substance to be measured at a measurement position in an elongated, substantially circularly cylindrical MAS rotor, wherein the front bearing has an opening (which can be closed by means of a closing device) for inserting an MAS rotor into the space between the base bearing and the front bearing.
A stator with a screwable closing device for installation in an NMR MAS probe head of the type stated in the introduction has been commercially available for some time, for example, from Prof. Ago Samoson, University of Technology, Tallinn, Estonia.
Nuclear magnetic resonance (=NMR) spectroscopy is a process for instrumental analysis with which, in particular, the chemical composition of measurement samples can be determined. Radio-frequency (RF) pulses are irradiated into the measurement sample, which is located in a strong, static magnetic field, and the electromagnetic reaction of the sample is measured.
To reduce spectral line broadening due to anisotropic interactions, it is known that an NMR sample can, during spectroscopic measurement, be tilted and rotated at the so-called “magic angle” of approx. 54.74° with respect to the static magnetic field (“MAS”=Magic Angle Spinning). For this purpose, the sample is filled into an MAS rotor. MAS rotors are cylindrical tubes open at one end, which are closed with a cap, wherein the cap is provided with vane elements (“impellers”). The MAS rotor is disposed in an MAS stator and the MAS rotor is made to rotate by gas pressure by means of the vane elements. The MAS rotor and MAS stator are collectively termed an MAS turbine.
During the NMR measurement, the MAS turbine is disposed in an NMR MAS probe head. The probe head has a cylindrical shield tube (also called “tube” for short) and usually a base box. The tube contains radio-frequency (=RF) electronic components, in particular RF resonator coils, and the MAS turbine, wherein the MAS turbine is disposed in the region of the tube end facing away from the base box. The probe head is typically inserted with its shield tube into the vertical room temperature bore of a superconducting magnet from below, then positioned, and held with hooks, supports, screws, or the like. The MAS turbine is then in the precise magnetic center of the magnet.
To replace an NMR sample or an MAS rotor filled with a substance to be measured on simple probe heads, it is necessary to remove the probe head from the magnet, i.e. to extract the probe head from the room temperature bore. For this purpose, the user kneels under the magnet, releases the supports and cable connections, and catches the probe head when it slides out of the magnet. Due to the eddy currents induced in the metal parts of the probe head, in particular in the shield tube, and the intrinsic weight of the probe head, removing the probe head, or indeed re-inserting it into the magnet, can require considerable exertion. To ensure safety, manufacturers of probe heads prescribe that the probe head should be removed by two people together. The rotor can then be replaced manually on the removed probe head. Re-shimming is usually necessary after the rotor has been replaced and the probe head therefore repositioned in the magnet, making the overall procedure very time-consuming.
DE 38 18 039 A1 discloses a rotatable sample magazine provided on the probe head in the immediate vicinity of the MAS stator such that the sample in the MAS stator can be replaced multiple times by the action of gas pressure without removing the probe head or the sample magazine from the interior of the magnet.
The technical poster of Shevgoor et al., discloses use of a lift system for MAS rotors. At the tube end of a probe head facing away from the base box, a transport tube is connected that extends through the room temperature bore of a magnet upward out of the magnet. By means of gas pressure, an MAS rotor can be transported through the transport tube into the MAS stator of the probe head mounted in the magnet, and an MAS rotor can also be transported out of the MAS stator upward out of the probe head.
Because the transport tube is routed through the room temperature bore, both the room temperature bore as well as regions above the magnet contain obstructive structures, which increases the complexity of the apparatus. Initial installation of the probe head is also rendered more difficult by the transport tube. The transport tube also has to be routed through the wall of the shield tube to the MAS stator, which makes RF shielding of the sample during NMR measurement more difficult.
In many cases, measurement of the sample under defined, extreme temperature conditions, in particular at cryogenic temperatures (−196° C. or lower) is desired. The interior of the shield tube is temperature controlled or cooled. In this case, the passage of the transport tube through the tube at the end farthest from the base box constitutes a thermal bridge that makes it more difficult to comply with defined, extreme temperature conditions.
DE 10 2008 054 152 B3 proposes a probe head with a base box and a tube attached to and protruding from the base box which permits a fast change between different MAS rotors while facilitating RF shielding and compliance with defined, extreme temperature conditions, wherein the MAS stator for receiving an MAS rotor is disposed inside the tube in the region of the tube end facing away from the base box. A transport tube is provided for pneumatically conveying an MAS rotor inside the transport tube, which extends in the interior of the tube from the base box to the MAS stator. However, in this conventional configuration, the front bearing does not have an opening, like on a generic probe head of the type defined in the introduction, which can be closed by means of a closing device for introducing an MAS rotor into the space between the base bearing and front bearing, so that changing rotors in the closed probe head is not possible. In particular, on MAS rotors with diameters<1.3 mm, closure at both ends is necessary to stabilize the rotation.
The generic probe head having the characteristics defined in the introduction and the stator offered by Prof. Ago Samoson with a screwable closing device cannot, for their part, be used with a rotor without having to remove the probe head from the magnet—with some effort.
This invention, on the other hand, has the object of providing an NMR MAS probe head of the type defined in the introduction with stabilized rotation with which a rotor change is possible without opening the probe head, and in which the probe head can remain in place in the magnet system.